U.S. patent application number 14/761236 was filed with the patent office on 2016-10-13 for electrolyte for lithium secondary battery and lithium secondary battery comprising same.
This patent application is currently assigned to SK INNOVATION CO., LTD.. The applicant listed for this patent is SK INNOVATION CO., LTD.. Invention is credited to Jin Su HAM, Jin Sung KIM, Jong Ho LIM, Seung Yon OH.
Application Number | 20160301103 14/761236 |
Document ID | / |
Family ID | 53371344 |
Filed Date | 2016-10-13 |
United States Patent
Application |
20160301103 |
Kind Code |
A1 |
KIM; Jin Sung ; et
al. |
October 13, 2016 |
ELECTROLYTE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY
BATTERY COMPRISING SAME
Abstract
Provided are an electrolyte for a high-voltage lithium secondary
battery and a high-voltage lithium secondary battery containing the
same, and more particularly, an electrolyte for a high-voltage
lithium secondary battery which may not be oxidized and decomposed
at the time of being kept at a high voltage and a high temperature
to prevent swelling of a battery through suppression of gas
generation, thereby having excellent high-temperature storage
characteristics and excellent discharge characteristics at a low
temperature while decreasing a thickness increase rate of the
battery, and a high-voltage lithium secondary battery containing
the same.
Inventors: |
KIM; Jin Sung; (Daejeon,
KR) ; OH; Seung Yon; (Daejeon, KR) ; LIM; Jong
Ho; (Daejeon, KR) ; HAM; Jin Su; (Daejeon,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SK INNOVATION CO., LTD. |
Seoul |
|
KR |
|
|
Assignee: |
SK INNOVATION CO., LTD.
|
Family ID: |
53371344 |
Appl. No.: |
14/761236 |
Filed: |
December 9, 2013 |
PCT Filed: |
December 9, 2013 |
PCT NO: |
PCT/KR2013/011347 |
371 Date: |
July 15, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 10/0525 20130101;
H01M 10/0567 20130101; H01M 10/052 20130101; H01M 10/0568 20130101;
Y02E 60/10 20130101; H01M 10/0569 20130101; H01M 2300/0025
20130101 |
International
Class: |
H01M 10/0567 20060101
H01M010/0567; H01M 10/0569 20060101 H01M010/0569; H01M 10/0525
20060101 H01M010/0525; H01M 10/0568 20060101 H01M010/0568 |
Claims
1. An electrolyte for a lithium secondary battery comprising: a
lithium salt; a non-aqueous organic solvent; and a multi-nitrile
compound represented by the following Chemical Formula 1:
##STR00009## in Chemical Formula 1, R.sub.1 to R.sub.3 are each
independently cyano, --(CH.sub.2).sub.a--CN,
--(CH.sub.2).sub.b--O--(CH.sub.2).sub.c--CN, or
(C1-C5)alkoxycarbonyl; R.sub.4 is hydrogen, cyano,
--(CH.sub.2).sub.a--CN, or
--(CH.sub.2).sub.b--O--(CH.sub.2).sub.c--CN; and a and c are each
independently integers of 2 to 10, and b is an integer of 1 to 10;
at least two of R.sub.1 to R.sub.4 being --(CH.sub.2).sub.m--CN or
--(CH.sub.2).sub.m--O--(CH.sub.2).sub.n--CN.
2. The electrolyte for a lithium secondary battery of claim 1,
wherein the multi-nitrile compound is represented by the following
Chemical Formula 2 or 3: ##STR00010## in Chemical Formulas 2 and 3,
R.sub.3 is cyano, --(CH.sub.2).sub.a--CN,
--(CH.sub.2).sub.b--O--(CH.sub.2).sub.c--CN, or
(C1-C5)alkoxycarbonyl; R.sub.4 is hydrogen, cyano,
--(CH.sub.2).sub.a--CN, or
--(CH.sub.2).sub.b--O--(CH.sub.2).sub.c--CN; and a and c are each
independently integers of 2 to 10, and b is an integer of 1 to
10.
3. The electrolyte for a lithium secondary battery of claim 2,
wherein the multi-nitrile compound is selected from multi-nitrile
compounds having the following structures. ##STR00011##
4. The electrolyte for a lithium secondary battery of claim 1,
wherein the multi-nitrile compound is contained at a content of 1
to 20 wt % based on a total weight of the electrolyte.
5. The electrolyte for a lithium secondary battery of claim 1,
further comprising one or two or more additives selected from the
group consisting of oxalatoborate based compounds, carbonate based
compounds substituted with fluorine, vinylidene carbonate based
compounds, and compounds containing a sulfinyl group.
6. The electrolyte for a lithium secondary battery of claim 5,
further comprising an additive selected from the group consisting
of lithium difluoro(oxalato)borate (LiFOB), lithium
bis(oxalato)borate (LiB(C.sub.2O.sub.4).sub.2, LiBOB),
fluoroethylene carbonate (FEC), vinylene carbonate (VC),
vinylethylene carbonate (VEC), divinyl sulfone, ethylene sulfite,
propylene sulfite, diallyl sulfonate, ethane sultone, propane
sultone (PS), butane sultone, ethene sultone, butene sultone, and
propene sultone (PRS).
7. The electrolyte for a lithium secondary battery of claim 5,
wherein the additive is contained at a content of 0.1 to 5.0 wt %
based on a total weight of the electrolyte.
8. The electrolyte for a lithium secondary battery of claim 1,
wherein the non-aqueous organic solvent is selected from cyclic
carbonate based solvents, linear carbonate based solvents, and a
mixed solvent thereof.
9. The electrolyte for a lithium secondary battery of claim 8,
wherein the cyclic carbonate is selected from the group consisting
of ethylene carbonate, propylene carbonate, butylene carbonate,
vinylene carbonate, vinylethylene carbonate, fluoroethylene
carbonate, and a mixture thereof, and the linear carbonate is
selected from the group consisting of dimethyl carbonate, diethyl
carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl
propyl carbonate, methyl isopropyl carbonate, ethyl propyl
carbonate, and a mixture thereof.
10. The electrolyte for a lithium secondary battery of claim 8,
wherein the non-aqueous organic solvent is a mixed solvent in which
the linear carbonate solvent and the cyclic carbonate solvent are
mixed at a mixed volume ratio of 1:1 to 9:1.
11. The electrolyte for a lithium secondary battery of claim 1,
wherein the lithium salt is one or two or more selected from the
group consisting of LiPF.sub.6, LiBF.sub.4, LiClO.sub.4,
LiSbF.sub.6, LiAsF.sub.6, LiN(SO.sub.2C.sub.2F.sub.5).sub.2,
LiN(CF.sub.3SO.sub.2).sub.2, LiN(SO.sub.3C.sub.2F.sub.5).sub.2,
LiCF.sub.3SO.sub.3, LiC.sub.4F.sub.9SO.sub.3,
LiC.sub.6H.sub.5SO.sub.3, LiSCN, LiAlO.sub.2, LiAlCl.sub.4,
LiN(C.sub.xF.sub.2x+1SO.sub.2)(C.sub.yF.sub.2y+1SO.sub.2) (here, x
and y are natural numbers), LiCl, LiI, and
LiB(C.sub.2O.sub.4).sub.2.
12. The electrolyte for a lithium secondary battery of claim 1,
wherein the lithium salt is contained at a concentration of 0.1 to
2.0 M.
13. A lithium secondary battery comprising the electrolyte for a
lithium secondary battery of claim 1.
Description
TECHNICAL FIELD
[0001] The present invention relates to an electrolyte for a
high-voltage lithium secondary battery and a high-voltage lithium
secondary battery containing the same, and more particularly, to an
electrolyte for a high-voltage lithium secondary battery, which may
not be oxidized and decomposed at the time of being kept at a high
voltage and a high temperature to prevent swelling of a battery
through suppression of gas generation, thereby having excellent
high-temperature storage characteristics and excellent discharge
characteristics at a low temperature while decreasing a thickness
increase rate of the battery, and a high-voltage lithium secondary
battery containing the same.
BACKGROUND ART
[0002] Recently, as a portable electronic device has widely spread,
in accordance with rapid miniaturization, lightness, and thinness
of the portable electronic device as described above, in a battery,
which is a power supply of the portable electronic device,
development of a secondary battery capable of having a small size
and a light weight, and being charged and discharged for a long
period of time, while having excellent high rate capability has
been urgently demanded.
[0003] Among the currently applied secondary batteries, a lithium
secondary battery, developed in the early 1990s, has been
spotlighted due to advantages such as a high operation voltage and
significantly high energy density as compared to conventional
batteries using an aqueous electrolyte such as a Ni-MH battery, a
Ni--Cd battery, and a lead sulfate battery, and the like. However,
in the lithium secondary battery as described above, there are
safety problems such as ignition and explosion, and the like,
caused by using a non-aqueous electrolyte. As a capacity density of
the battery is increased, this problem becomes more severe.
[0004] In a non-aqueous electrolyte secondary battery, there is a
serious problem such as safety deterioration of the battery
generated at the time of continuous charge. One of the causes
affecting safety of the battery is heat generation due to collapse
of a cathode structure. An operation principle thereof is as
follows. That is, a cathode active material of a non-aqueous
electrolyte battery is composed of lithium, a lithium containing
metal oxide capable of intercalating and releasing lithium ions,
and/or the like, and as a large amount of lithium is detached at
the time of over-charge, a structure of the cathode active material
as described above is changed to a thermally unstable structure. In
this over-charge state, when a battery temperature reaches a
critical temperature due to external physical impact, for example,
exposure to a high temperature, or the like, oxygen is released
from the cathode active material having an unstable structure, and
the released oxygen generates an exothermic decomposition reaction
with an electrolyte solvent, or the like. Particularly, since
combustion of the electrolyte is further accelerated by oxygen
released from a cathode, the battery may be ignited and ruptured
due to thermal runaway caused by a series of exothermic reactions
as described above.
[0005] In order to suppress the above-mentioned ignition or rupture
due to an increase in a temperature in the battery, a method of
adding an aromatic compound to the electrolyte as a redox shuttle
additive has been used. For example, a non-aqueous lithium ion
battery capable of preventing over-charge current and a thermal
runaway phenomenon caused by the over-charge current by using an
aromatic compound such as biphenyl has been disclosed in Japanese
Patent No. 2002-260725. In addition, a method of improving safety
of a battery by adding a small amount of an aromatic compound such
as biphenyl, 3-chlorothiophene, or the like, to increase an
internal resistance by electrochemical neutralization in an
abnormal over-voltage state has been disclosed in U.S. Pat. No.
5,879,834.
[0006] However, in the case of using the additive such as biphenyl,
or the like, there are problems in that when a relatively high
voltage is locally generated in a general operation voltage, the
additive is gradually decomposed during a charge and discharge
process, or when the battery is discharged at a high temperature
for a long period of time, an amount of biphenyl, or the like, may
be gradually decreased, such that safety may not be secured after
300 charge and discharge cycles. In addition, there is a problem in
storage characteristics, or the like.
[0007] Meanwhile, as a method of increasing an electricity charge
amount for a small size and high capacity of the battery, a high
voltage battery (4.4V system) has been continuously studied and
developed. Even in the same battery system, when a charge voltage
is increased, a charge amount is generally increased. However,
safety problems such as decomposition of the electrolyte, a
shortage of a space for lithium intercalation, a risk due to a
potential rise of an electrode, or the like, may occur. Therefore,
in order to manufacture a battery that may be used at a high
voltage, overall conditions are managed with a system so that a
large standard reduction potential difference between an anode
active material and a cathode active material may be easily
maintained, and an electrolyte is not decomposed at this
voltage.
[0008] Considering this point of the high voltage battery, it may
be appreciated that in the case of using existing over-charge
preventing agents such as biphenyl (BP) or cyclohexylbenzene (CHB)
used in a general lithium ion battery, even during a normal charge
and discharge operation, large amounts of these over-charge
preventing agents may be decomposed, and characteristics of the
battery may be rapidly deteriorated even at a slightly high
temperature, such that a life cycle of the battery may be
decreased. Further, in the case of using a generally used
non-aqueous carbonate based solvent as an electrolyte, when a
battery is charged at a voltage higher than 4.2V, which is a
general charge potential, oxidizing power may be increased, such
that as charge and discharge cycles are performed, a decomposition
reaction of the electrolyte is carried out, such that life cycle
characteristics may be rapidly deteriorated.
[0009] Therefore, a method for improving safety and capacity of a
battery at the time of high-temperature storage without
deteriorating life cycle characteristics of a high voltage battery
(4.4V system) has been continuously demanded.
DISCLOSURE
Technical Problem
[0010] An object of the present invention is to provide an
electrolyte for a high-voltage lithium secondary battery capable of
significantly decreasing a swelling phenomenon of a battery due to
oxidation/decomposition of the electrolyte at a high voltage state
while properly maintaining basic performance such as high rate
charge and discharge characteristics, life cycle characteristics,
and the like, to thereby have excellent high-temperature storage
characteristics and discharge characteristics at a low temperature,
and a high-voltage lithium secondary battery containing the
same.
Technical Solution
[0011] In one general aspect, an electrolyte for a high-voltage
lithium secondary battery contains:
[0012] a lithium salt;
[0013] a non-aqueous organic solvent; and
[0014] a multi-nitrile compound represented by the following
Chemical Formula 1:
##STR00001##
[0015] in Chemical Formula 1, R.sub.1 to R.sub.3 are each
independently cyano, --(CH.sub.2).sub.a--CN,
--(CH.sub.2).sub.b--O--(CH.sub.2).sub.c--CN, or
(C1-C5)alkoxycarbonyl; R.sub.4 is hydrogen, cyano,
--(CH.sub.2).sub.a--CN, or
--(CH.sub.2).sub.b--O--(CH.sub.2).sub.c--CN; and a and c are each
independently integers of 2 to 10, and b is an integer of 1 to 10;
at least two of R.sub.1 to R.sub.4 being --(CH.sub.2).sub.m--CN or
--(CH.sub.2).sub.m--O--(CH.sub.2).sub.n--CN.
[0016] The multi-nitrile compound may be represented by the
following Chemical Formula 2 or 3:
##STR00002##
[0017] in Chemical Formulas 2 and 3, R.sub.3 is cyano,
--(CH.sub.2).sub.a--CN,
--(CH.sub.2).sub.b--O--(CH.sub.2).sub.c--CN, or
(C1-C5)alkoxycarbonyl; R.sub.4 is hydrogen, cyano,
--(CH.sub.2).sub.a--CN, or --(CH.sub.2).sub.b--O--(CH.sub.2)--CN;
and a and c are each independently integers of 2 to 10, and b is an
integer of 1 to 10.
[0018] The multi-nitrile compound may be selected from
multi-nitrile compounds having the following structures.
##STR00003##
[0019] The multi-nitrile compound may be contained at a content of
1 to 20 wt % based on a total weight of the electrolyte.
[0020] The electrolyte may further contain one or two or more
additives selected from the group consisting of oxalatoborate based
compounds, carbonate based compounds substituted with fluorine,
vinylidene carbonate based compounds, and compounds containing a
sulfinyl group.
[0021] The electrolyte may further contain an additive selected
from the group consisting of lithium difluoro(oxalato)borate
(LiFOB), lithium bis(oxalato)borate (LiB(C.sub.2O.sub.4).sub.2,
LiBOB), fluoroethylene carbonate (FEC), vinylene carbonate (VC),
vinylethylene carbonate (VEC), divinyl sulfone, ethylene sulfite,
propylene sulfite, diallyl sulfonate, ethane sultone, propane
sultone (PS), butane sultone, ethene sultone, butene sultone, and
propene sultone (PRS).
[0022] The additive may be contained at a content of 0.1 to 5 wt %
based on a total weight of the electrolyte.
[0023] The non-aqueous organic solvent may be selected from cyclic
carbonate based solvents, linear carbonate based solvents, and a
mixed solvent thereof, wherein the cyclic carbonate may be selected
from the group consisting of ethylene carbonate, propylene
carbonate, butylene carbonate, vinylene carbonate, vinylethylene
carbonate, fluoroethylene carbonate, and a mixture thereof, and the
linear carbonate may be selected from the group consisting of
dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl
methyl carbonate, methyl propyl carbonate, methyl isopropyl
carbonate, ethyl propyl carbonate, and a mixture thereof.
[0024] The non-aqueous organic solvent may be a mixed solvent in
which the linear carbonate solvent and the cyclic carbonate solvent
are mixed at a mixed volume ratio of 9:1 to 1:1.
[0025] The lithium salt may be one or two or more selected from the
group consisting of LiPF.sub.6, LiBF.sub.4, LiClO.sub.4,
LiSbF.sub.6, LiAsF.sub.6, LiN(SO.sub.2C.sub.2F.sub.5).sub.2,
LiN(CF.sub.3SO.sub.2).sub.2, LiN(SO.sub.3C.sub.2F.sub.5).sub.2,
LiCF.sub.3SO.sub.3, LiC.sub.4F.sub.9SO.sub.3,
LiC.sub.6H.sub.5SO.sub.3, LiSCN, LiAlO.sub.2, LiAlCl.sub.4,
LiN(C.sub.xF.sub.2x+1SO.sub.2) (C.sub.yF.sub.2y+1SO.sub.2) (here, x
and y are natural numbers), LiCl, LiI, and
LiB(C.sub.2O.sub.4).sub.2.
[0026] The lithium salt may be contained at a concentration of 0.1
to 2.0 M.
[0027] In another general aspect, a lithium secondary battery
contains the electrolyte for a high-voltage lithium secondary
battery as described above.
Advantageous Effects
[0028] An electrolyte for a high-voltage lithium secondary battery
according to the present invention contains a multi-nitrile
compound having a structure in which at least three of four
substituents substituted at a central carbon atom are substituents
except for hydrogen, and at the same time, at least two thereof are
nitrile groups, that is, cyanoalkyl groups or cyanoalkyloxyalkyl
groups, such that a swelling phenomenon of a battery due to
oxidation/decomposition of the electrolyte in a high voltage state
may be significantly decreased, and thus, the electrolyte may have
excellent discharge characteristics even at a low temperature as
well as excellent high-temperature storage characteristics.
[0029] Further, a high-voltage lithium secondary battery containing
the electrolyte for a high-voltage lithium secondary battery
according to the present invention may significantly decrease a
swelling phenomenon of the battery due to oxidation/decomposition
of the electrolyte at a high voltage state while properly
maintaining basic performance such as high efficiency charge and
discharge characteristics, life cycle characteristics, and the
like, thereby having and discharge characteristics at a low
temperature as well as excellent high-temperature storage
characteristics.
DESCRIPTION OF DRAWINGS
[0030] The above and other objects, features and advantages of the
present invention will become apparent from the following
description of preferred embodiments given in conjunction with the
accompanying drawings, in which:
[0031] FIG. 1 is a graph illustrating measurement results of
oxidative decomposition potentials of Examples 1 to 4 and
Comparative Example 1.
BEST MODE
[0032] Hereinafter, the present invention will be described in more
detail. Here, technical terms and scientific terms used in the
present specification have the general meaning understood by those
skilled in the art to which the present invention pertains unless
otherwise defined, and a description for the known function and
configuration unnecessarily obscuring the gist of the present
invention will be omitted in the following description.
[0033] The present invention relates to an electrolyte for a
high-voltage lithium secondary battery for providing a battery
having excellent high-temperature storage characteristics and life
cycle characteristics while securing safety of the battery at a
high voltage state.
[0034] The present invention provides an electrolyte for a
high-voltage lithium secondary battery containing a lithium salt; a
non-aqueous organic solvent; and a multi-nitrile compound
represented by the following Chemical Formula 1:
##STR00004##
[0035] in Chemical Formula 1, R.sub.1 to R.sub.3 are each
independently cyano, --(CH.sub.2).sub.a--CN,
--(CH.sub.2).sub.b--O--(CH.sub.2).sub.c--CN, or
(C1-C5)alkoxycarbonyl; R.sub.4 is hydrogen, cyano,
--(CH.sub.2).sub.a--CN, or
--(CH.sub.2).sub.b--O--(CH.sub.2).sub.c--CN; and a and c are each
independently integers of 2 to 10, and b is an integer of 1 to 10;
at least two of R.sub.1 to R.sub.4 being --(CH.sub.2).sub.m--CN or
--(CH.sub.2).sub.m--O--(CH.sub.2).sub.n--CN.
[0036] An effect as an electrolyte additive may be changed
depending on a shape of a structure introduced between two nitrile
groups, and in the case of a compound having a linear aliphatic
hydrocarbon group introduced therein, in which another substituent
except for hydrogen atoms is not introduced between two nitrile
groups, atoms may freely move in molecules, such that permittivity
may be relatively decreased, and thus the electrolyte may be easily
oxidized and decomposed at a high voltage. Therefore, in the case
of adding the compound having the linear aliphatic hydrocarbon
group introduced therein, in which another substituent except for
hydrogen is not introduced between two nitrile groups, to the
electrolyte, the electrolyte may be easily oxidized and decomposed
at a high voltage, thereby causing side reactions in a battery.
However, the electrolyte for a high-voltage lithium secondary
battery according to the present invention contains the
multi-nitrile compound having a structure in which at least three
of four substituents substituted at a central carbon atom are
substituents except for hydrogen, and at the same time, at least
two thereof are nitrile groups, that is, cyanoalkyl groups or
cyanoalkyloxyalkyl groups, such that side reactions in a battery
may be suppressed. Therefore, a swelling phenomenon of the battery
due to oxidation/decomposition of the electrolyte in a high voltage
state may be significantly decreased, such that the battery may
have excellent discharge characteristics even at a low temperature
as well as excellent high-temperature storage characteristics.
[0037] In the electrolyte for a high-voltage lithium secondary
battery according to an exemplary embodiment of the present
invention, the multi-nitrile compound may be represented by the
following Chemical Formula 2 or 3:
##STR00005##
[0038] in Chemical Formulas 2 and 3, R.sub.3 is cyano,
--(CH.sub.2).sub.a--CN,
--(CH.sub.2).sub.b--O--(CH.sub.2).sub.a--CN, or
(C1-C5)alkoxycarbonyl; R.sub.4 is hydrogen, cyano,
--(CH.sub.2).sub.a--CN, or
--(CH.sub.2).sub.b--O--(CH.sub.2).sub.c--CN; and a and c are each
independently integers of 2 to 10, and b is an integer of 1 to
10.
[0039] In the electrolyte for a high-voltage lithium secondary
battery according to the embodiment of the present invention, most
preferably, the multi-nitrile compound may be selected from
multi-nitrile compounds having the following structures.
##STR00006##
[0040] In the electrolyte for a high-voltage lithium secondary
battery according to an exemplary embodiment of the present
invention, the multi-nitrile compound represented by Chemical
Formula 1 may be contained at a content of 1 to 20 wt %, more
preferably 1 to 15 wt % based on a total weight of the electrolyte
for a secondary battery. When the content of the multi-nitrile
compound represented by Chemical Formula 1 is less than 1 wt. %,
addition effects such as suppression of the swelling phenomenon of
the battery during high-temperature storage, improvement of a
capacity retention rate, or the like, are not exhibited, and an
effect of improving discharge capacity, output, or the like, of the
lithium secondary battery may be insufficient, and when the content
is more than 20 wt %, a life cycle, or the like, is rapidly
deteriorated, such that characteristics of the lithium secondary
battery may be rather deteriorated.
[0041] In the electrolyte for a high-voltage lithium secondary
battery according to an exemplary embodiment of the present
invention, the electrolyte may further contain one or two or more
additives selected from the group consisting of oxalatoborate based
compounds, carbonate based compounds substituted with fluorine,
vinylidene carbonate based compounds, and compounds containing a
sulfinyl group as a life cycle improving additive for improving the
life cycle of the battery.
[0042] The oxalatoborate based compound may be a compound
represented by the following Chemical Formula 4 or lithium
bis(oxalato)borate (LiB(C.sub.2O.sub.4).sub.2, LiBOB).
##STR00007##
[0043] (In Chemical Formula 4, R.sub.11 and R.sub.12 are each
independently a halogen atom or a halogenated (C1 to C10)alkyl
group.)
[0044] Specific examples of the oxalatoborate based additive may
include lithium difluoro(oxalato)borate (LiB(C.sub.2O.sub.4)
F.sub.2, LiFOB), lithium bis(oxalato)borate
(LiB(C.sub.2O.sub.4).sub.2, LiBOB), or the like.
[0045] The carbonate based compound substituted with fluorine may
be fluoroethylene carbonate (FEC), difluoroethylene carbonate
(DFEC), fluorodimethyl carbonate (FDMC), fluoroethyl methyl
carbonate (FEMC), or a combination thereof.
[0046] The vinylidene carbonate based compound may be vinylene
carbonate (VC), vinyl ethylene carbonate (VEC), or a mixture
thereof.
[0047] The compound containing a sulfinyl (S.dbd.O) group may be
sulfone, sulfite, sulfonate, and sultone (cyclic sulfonate), and
the compound may be used alone or a mixture thereof may be used. In
detail, the sulfone may be represented by the following Chemical
Formula 5 and be divinyl sulfone. The sulfite may be represented by
the following Chemical Formula 6 and be ethylene sulfite or
propylene sulfite. Sulfonate may be represented by the following
Chemical Formula 7 and be diallyl sulfonate. In addition,
non-restrictive examples of sultone may include ethane sultone,
propane sultone, butane sultone, ethene sultone, butene sultone,
propene sultone, and the like.
##STR00008##
[0048] (In Chemical Formulas 5 to 7, R.sub.13 and R.sub.14 are each
independently hydrogen, a halogen atom, a (C1-C10)alkyl group, a
(C2-C10)alkenyl group, a (C1-C10)alkyl group substituted with
halogen, or a (C2-C10)alkenyl group substituted with halogen.)
[0049] More preferably, the electrolyte for a high-voltage lithium
secondary battery according to an exemplary embodiment of the
present invention may further contain an additive selected from the
group consisting of lithium difluoro(oxalato)borate (LiFOB),
lithium bis(oxalato)borate (LiB(C.sub.2O.sub.4).sub.2, LiBOB),
fluoroethylene carbonate (FEC), vinylene carbonate (VC),
vinylethylene carbonate (VEC), divinyl sulfone, ethylene sulfite,
propylene sulfite, diallyl sulfonate, ethane sultone, propane
sultone (PS), butane sultone, ethene sultone, butene sultone, and
propene sultone (PRS).
[0050] In the electrolyte for a high-voltage lithium secondary
battery according to an exemplary embodiment of the present
invention, a content of the additive is not particularly limited,
but in order to improve the life cycle of the battery, the additive
may be contained in the electrolyte for a secondary battery at a
content of 0.1 to 5 wt %, more preferably 0.1 to 3 wt % based on a
total weight of the electrolyte.
[0051] In the electrolyte for a high-voltage lithium secondary
battery according to an exemplary embodiment of the present
invention, the non-aqueous organic solvent may include carbonate,
ester, ether, or ketone alone, or a mixed solvent thereof, but it
is preferable that the non-aqueous organic solvent is selected from
cyclic carbonate based solvents, linear carbonate based solvents,
and a mixed solvent thereof. It is most preferable to use a mixture
of the cyclic carbonate based solvent and the linear carbonate
based solvent. The cyclic carbonate based solvent may sufficiently
dissociate lithium ions due to large polarity, but has a
disadvantage in that ion conductivity thereof is small due to a
large viscosity. Therefore, characteristics of the lithium
secondary battery may be optimized by mixing the linear carbonate
solvent having a small polarity and a low viscosity with the cyclic
carbonate solvent.
[0052] The cyclic carbonate based solvent may be selected from the
group consisting of ethylene carbonate, propylene carbonate,
butylene carbonate, vinylene carbonate, vinylethylene carbonate,
fluoroethylene carbonate, and a mixture thereof, and the linear
carbonate based solvent may be selected from the group consisting
of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl
methyl carbonate, methyl propyl carbonate, methyl isopropyl
carbonate, ethyl propyl carbonate, and a mixture thereof.
[0053] In the electrolyte for a high-voltage lithium secondary
battery according to an exemplary embodiment of the present
invention, in the non-aqueous organic solvent, which is the mixed
solvent of the cyclic carbonate based solvent and the linear
carbonate based solvent, a mixed volume ratio of the linear
carbonate based solvent and the cyclic carbonate based solvent may
be 1:1 to 9:1, preferably 1.5:1 to 4:1.
[0054] In the electrolyte for a high-voltage lithium secondary
battery according to an exemplary embodiment of the present
invention, the lithium salt may be one or two or more selected from
the group consisting of LiPF.sub.6, LiBF.sub.4, LiClO.sub.4,
LiSbF.sub.6, LiAsF.sub.6, LiN(SO.sub.2C.sub.2F.sub.5).sub.2,
LiN(CF.sub.3SO.sub.2).sub.2, LiN(SO.sub.3C.sub.2F.sub.5).sub.2,
LiCF.sub.3SO.sub.3, LiC.sub.4F.sub.9SO.sub.3,
LiC.sub.6H.sub.5SO.sub.3, LiSCN, LiAlO.sub.2, LiAlCl.sub.4,
LiN(C.sub.xF.sub.2x+1SO.sub.2) (C.sub.yF.sub.2y+1SO.sub.2) (here, x
and y are natural numbers), LiCl, LiI, and
LiB(C.sub.2O.sub.4).sub.2, but is not limited thereto.
[0055] The lithium salt may be used in a concentration range of
preferably 0.1 to 2.0 M, and more preferably, 0.7 to 1.6 M. In the
case in which the concentration of the lithium salt is less than
0.1 M, conductivity of the electrolyte is decreased, such that
performance of the electrolyte is deteriorated, and in the case in
which the concentration is more than 2.0 M, the viscosity of the
electrolyte is increased, such that movability of the lithium ion
may be decreased. The lithium salt acts as a supply source of the
lithium ion in the battery to enable a basic operation of the
lithium secondary battery.
[0056] Since the electrolyte for a high-voltage lithium secondary
battery according to an exemplary embodiment of the present
invention is generally stable in a temperature range of -20 to
60.degree. C., and maintains electrochemically stable
characteristics thereof even at a voltage of 4.4 V, the electrolyte
may be applied to all of the lithium secondary batteries such as a
lithium ion battery, a lithium polymer battery, and the like.
[0057] In addition, the present invention provides a high-voltage
lithium secondary battery containing the electrolyte for a
high-voltage lithium secondary battery.
[0058] A non-restrictive example of the secondary battery may
include a lithium metal secondary battery, a lithium ion secondary
battery, a lithium polymer secondary battery, a lithium ion polymer
secondary battery, or the like.
[0059] The high-voltage lithium secondary battery manufactured
using the electrolyte for a high-voltage lithium secondary battery
according to the present invention has low-temperature discharge
efficiency of 70 or more and high-temperature storage efficiency of
75% or more, and at the time of keeping the high-voltage lithium
secondary battery at a high temperature for a long period of time,
a thickness increase rate of the battery is significantly low (4 to
14%).
[0060] The high-voltage lithium secondary battery according to the
present invention includes a cathode and an anode.
[0061] It is preferable that the cathode contains a cathode active
material capable of intercalating and deintercalating a lithium
ion, and it is preferable that the cathode active material as
described above is a complex metal oxide of at least one kind
selected from cobalt, manganese, and nickel and lithium. An
employment ratio between metals may be various, and an element
selected from the group consisting of Mg, Al, Co, K, Na, Ca, Si,
Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Cr, Fe, Sr, V, and rare earth
elements may be further contained in addition to the
above-mentioned metals. As a specific example of the cathode active
material, a compound represented by any one of the following
Chemical Formulas may be used.
[0062] Li.sub.aA.sub.1-bB.sub.bD.sub.2 (0.90.ltoreq.a.ltoreq.1.8
and 0.ltoreq.b.ltoreq.0.5); Li.sub.aE.sub.1-bB.sub.bO.sub.2-c
D.sub.c (0.90.ltoreq.a.ltoreq.1.8, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.05); LiE.sub.2-bB.sub.bO.sub.4-cD.sub.c
(0.ltoreq.b.ltoreq.0.5, 0.ltoreq.c.ltoreq.0.05);
Li.sub.aNi.sub.1-b-cCo.sub.bB.sub.cD.sub..alpha.
(0.90.ltoreq.a.ltoreq.1.8, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.05, 0.ltoreq..alpha..ltoreq.2);
Li.sub.aNi.sub.1-b-cCo.sub.bB.sub.cO.sub.2-.alpha.F.sub..alpha.
(0.90.ltoreq.a.ltoreq.1.8, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.05, 0.ltoreq..alpha..ltoreq.2);
Li.sub.aNi.sub.1-b-cCo.sub.bB.sub.cO.sub.2-.alpha.F.sub.2
(0.90.ltoreq.a.ltoreq.1.8, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.05, 0.ltoreq..alpha..ltoreq.2);
Li.sub.aNi.sub.1-b-cMn.sub.bB.sub.cD.sub..alpha.
(0.90.ltoreq.a.ltoreq.1.8, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.05, 0<.alpha..ltoreq.2);
Li.sub.aNi.sub.1-b-cMn.sub.bB.sub.cO.sub.2-.alpha.F.sub..alpha.
(0.90.ltoreq.a.ltoreq.1.8, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.05, 0.ltoreq..alpha..ltoreq.2);
Li.sub.aNi.sub.1-b-cMn.sub.bB.sub.cO.sub.2-.alpha.F.sub.2
(0.90.ltoreq.a.ltoreq.1.8, 0.ltoreq.b.ltoreq.0.5,
0.ltoreq.c.ltoreq.0.05, 0.ltoreq..alpha..ltoreq.2);
Li.sub.aNi.sub.bE.sub.cG.sub.dO.sub.2 (0.90.ltoreq.a.ltoreq.1.8,
0.ltoreq.b.ltoreq.0.9, 0.ltoreq.c.ltoreq.0.5,
0.001.ltoreq.d.ltoreq.0.1);
Li.sub.aNi.sub.bCo.sub.cMn.sub.dGeO.sub.2
(0.90.ltoreq.a.ltoreq.1.8, 0.ltoreq.b.ltoreq.0.9,
0.ltoreq.c.ltoreq.0.5, 0.ltoreq.d.ltoreq.0.5,
0.001.ltoreq.e.ltoreq.0.1); Li.sub.aNiG.sub.bO.sub.2
(0.90.ltoreq.a.ltoreq.1.8, 0.001.ltoreq.b.ltoreq.0.1);
Li.sub.aCoG.sub.bO.sub.2 (0.90.ltoreq.a.ltoreq.1.8,
0.001.ltoreq.b.ltoreq.0.1); Li.sub.aMnG.sub.bO.sub.2
(0.90.ltoreq.a.ltoreq.1.8, 0.001.ltoreq.b.ltoreq.0.1);
Li.sub.aMn.sub.2G.sub.bO.sub.4 (0.90.ltoreq.a.ltoreq.1.8, 0.001
.ltoreq.0.1); QO.sub.2; QS.sub.2; LiQS.sub.2; V.sub.2O.sub.5;
LiV.sub.2O.sub.5; LiIO.sub.2; LiNiVO.sub.4;
Li.sub.(3-f)J.sub.2(PO.sub.4).sub.3 (0.ltoreq.f.ltoreq.2);
Li.sub.(3-F)Fe.sub.2(PO.sub.4).sub.3 (0.ltoreq.f.ltoreq.2); and
LiFePO.sub.4.
[0063] In the Chemical Formulas, A may be Ni, Co, Mn, or a
combination thereof; B may be Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a
rare earth element, or a combination thereof; D may be O, F, S, P,
or a combination thereof; E may be Co, Mn, or a combination
thereof; F may be F, S, P, or a combination thereof; G may be Al,
Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; Q may be
Ti, Mo, Mn, or a combination thereof; I may be Cr, V, Fe, Sc, Y, or
a combination thereof; and J may be V, Cr, Mn, Co, Ni, Cu, or a
combination thereof.
[0064] The anode contains an anode active material capable of
intercalating and deintercalating the lithium ion, and as this
anode active material, a carbon material such as crystalloid
carbon, amorphous carbon, carbon complex, a carbon fiber, or the
like, a lithium metal, an alloy of lithium and another element, or
the like, may be used. Examples of the amorphous carbon may include
hard carbon, coke, mesocarbon microbead (MCMB) sintered at a
temperature of 1500.degree. C. or less, mesophase pitch-based
carbon fiber (MPCF), and the like. Examples of the crystalloid
carbon may include graphite based materials, more specifically,
natural graphite, graphitized coke, graphitized MCMB, graphitized
MPCF, and the like. As the carbon material, a material of which a
d002 interplanar distance is 3.35 to 3.38 .ANG., and a crystallite
size Lc measured by X-ray diffraction is at least 20 nm or more may
be preferable. Another element forming an alloy with lithium may be
aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin,
gallium, or indium.
[0065] The cathode or anode may be prepared by dispersing an
electrode active material, a binder, and a conductive material, and
if necessary, a thickener, in a solvent to prepare an electrode
slurry composition, and applying this electrode slurry composition
onto an electrode current collector. As a cathode current
collector, aluminum, an aluminum alloy, or the like, may be
generally used, and as an anode current collector, copper, a copper
alloy, or the like, may be generally used. The cathode current
collector and the anode current collector have a foil or mesh
shape.
[0066] The binder is a material playing a role in paste formation
of the active material, adhesion between the active materials,
adhesion with the current collector, and a buffering effect on
expansion and contraction of the active material, and the like.
Examples of the binder may include polyvinylidene fluoride (PVdF),
a polyhexafluoropropylene-polyvinylidene fluoride copolymer
(PVdF/HFP), poly(vinylacetate), polyvinyl alcohol,
polyethyleneoxide, polyvinylpyrrolidone, alkylated
polyethyleneoxide, polyvinyl ether, poly(methylmethacrylate),
poly(ethylacrylate), polytetrafluoroethylene, polyvinylchloride,
polyacrylonitrile, polyvinylpyridine, styrene-butadiene rubber,
acrylonitrile-butadiene rubber, and the like. A content of the
binder is 0.1 to 30 wt %, preferably 1 to 10 wt % based on the
electrode active material. In the case in which the content of the
binder is excessively low, adhesive force between the electrode
active material and the current collector may become insufficient,
and in the case in which the content is excessively high, adhesive
force may be improved, but a content of the electrode active
material is decreased in accordance with the content of the binder,
which is disadvantageous in allowing the battery to have high
capacity.
[0067] The conductive material is used to impart conductivity to
the electrode, and any electronic conductive material may be used
as long as it does not cause a chemical change in a battery to be
configured. At least one selected from the group consisting of a
graphite based conductive material, a carbon black based conductive
material, and a metal or metal compound based conductive material
may be used. Examples of the graphite based conductive material may
include artificial graphite, natural graphite, and the like,
examples of the carbon black based conductive material may include
acetylene black, Ketjen black, Denka black, thermal black, channel
black, and the like, and examples of the metal or metal compound
based conductive material may include tin, tin oxide, tin phosphate
(SnPO.sub.4), titanium oxide, potassium titanate, a perovskite
material such as LaSrCoO.sub.3 and LaSrMnO.sub.3. However, the
conductive material is not limited thereto.
[0068] A content of the conductive material is preferably 0.1 to 10
wt % based on the electrode active material. In the case in which
the content of the conductive material is less than 0.1 wt %,
electrochemical properties may be deteriorated, and in the case in
which the content is more than 10 wt %, energy density per weight
may be decreased.
[0069] Any thickener may be used without limitation as long as it
may serve to adjust a viscosity of the active material slurry, but
for example, carboxymethyl cellulose, hydroxymethyl cellulose,
hydroxyethyl cellulose, hydroxypropyl cellulose, or the like, may
be used.
[0070] As the solvent in which the electrode active material, the
binder, the conductive material, and the like, are dispersed, a
non-aqueous solvent or aqueous solvent may be used. Examples of the
non-aqueous solvent may include N-methyl-2-pyrrolidone (NMP),
dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine,
ethyleneoxide, tetrahydrofuran, or the like.
[0071] The high-voltage lithium secondary battery according to the
present invention may include a separator preventing a
short-circuit between the cathode and the anode and providing a
movement path of the lithium ion. As the separator as described
above, a polyolefin based polymer membrane made of polypropylene,
polyethylene, polyethylene/polypropylene,
polyethylene/polypropylene/polyethylene,
polypropylene/polyethylene/polypropylene, or the like, or a
multilayer thereof, a micro-porous film, and woven fabric and
non-woven fabric may be used. In addition, a film in which a resin
having excellent stability is coated on a porous polyolefin film
may be used.
[0072] The high-voltage lithium secondary battery according to the
present invention may have various shapes such as a cylindrical
shape, a pouch shape, in addition to an angular shape.
[0073] Hereinafter, Examples and Comparative Examples of the
present invention will be described. However, the following Example
is only a preferable example of the present invention, and the
present invention is not limited thereto. Under the assumption that
the lithium salt is entirely dissociated so that a concentration of
lithium ion becomes 1 M, a base electrolyte may be formed by
dissolving a corresponding amount of the lithium salt such as
LiPF.sub.6 in a basic solvent so as to have a concentration of 1
M.
Preparation Example 1
Synthesis of 3-[3-(2-cyanoethoxy)-2,2-bis-(2-cyanoethoxymethyl)
propoxy]propionitrile (hereinafter, referred to as `PHE22`)
[0074] 21 g of pentaerythritol and 17 mL of 40% potassium hydroxide
aqueous solution were added to 75 mL of xylene. 133 g of
acrylonitrile was slowly added to this solution at room temperature
for 1 hour and then, stirred at room temperature for 3 hours. After
adding toluene thereto, an organic layer was washed three times
with 5% sodium chloride aqueous solution and hydrochloric acid
aqueous solution. The organic layer was dried by adding magnesium
sulfate and filtered, and a solvent was removed by distillation
under reduced pressure, thereby obtaining 52 g of
3-[3-(2-cyanoethoxy)-2,2-bis-(2-cyanoethoxymethyl)propoxy]propionitr-
ile as a colorless liquid.
[0075] .sup.1H NMR (CDCl.sub.3, 500 MHz) .delta. 3.67 (t, 8H), 3.49
(S, 8H), 2.61 (t, 8H)
Preparation Example 2
Synthesis of diethyl 2,2-bis(2-cyanoethyl)malonate (hereinafter,
referred to as `PHE24`)
[0076] 50 g of diethyl malonate and triton-B (40% in methanol, 6.5
g) were put into 60 mL of dioxane, and then, 33 g of acetonitrile
was slowly added thereto. After the mixture was stirred at
60.degree. C. for 12 hours, a temperature was lowered to room
temperature. The mixture was neutralized with 0.1N hydrochloric
acid aqueous solution, and then, the reactant was added to ice
water. The precipitated product was filtered and then,
recrystallized with ethanol, thereby obtaining 74 g of diethyl
2,2-bis(2-cyanoethyl)malonate as a white solid product.
[0077] Melting Point: 62.2-63.5
[0078] .sup.1H NMR (CDCl.sub.3, 500 MHz) .delta. 4.10 (t, 4H), 2.47
(t, 4H), 2.26 (t, 4H), 1.20 (t, 6H)
Preparation Example 3
Synthesis of 4,4-dicyano-heptanedinitrile (hereinafter, referred to
as `PHE25`)
[0079] 56 g of acrylonitrile was slowly added to an aqueous
solution obtained by dissolving 30 g of malononitrile and 2.6 g of
potassium hydroxide in 200 mL of water for 1 hour, and then stirred
at room temperature for 2 hours. After the reaction was terminated
using IN hydrochloric acid aqueous solution, an organic layer was
extracted with ethylacetate, and then washed three times with
water. The ethylacetate solvent in the organic solvent layer was
removed by distillation under reduced pressure. A solid produced by
removing the solvent was dried in a vacuum oven, thereby obtaining
43 g of 4,4-dicyano-heptanedinitrile.
[0080] .sup.1H NMR (CDCl.sub.3, 500 MHz) .delta. 2.60 (t, 4H), 2.30
(t, 4H)
Preparation Example 4
Synthesis of 1,2,3-tris-(P-cyanoethoxy)-propane (hereinafter,
referred to as `PHE31`)
[0081] 14 g of glycerol and 17 mL of 40% potassium hydroxide
aqueous solution were added to 75 mL of xylene. 133 g of
acrylonitrile was slowly added to this solution at room temperature
for 1 hour and then, stirred at room temperature for 3 hours. After
adding toluene thereto, an organic layer was washed three times
with 5% sodium chloride aqueous solution and hydrochloric acid
aqueous solution. The organic layer was dried by adding magnesium
sulfate and filtered, and then, a solvent was removed by
distillation under reduced pressure, thereby obtaining a colorless
liquid. 35 g of purified 1,2,3-tris-(P-cyanoethoxy)-propane was
obtained by distillation under reduced pressure.
[0082] .sup.1H NMR (CDCl.sub.3, 500 MHz) .delta. 3.87 (m, 1H), 3.72
(m, 6H), 3.62 (m, 6H), 2.64 (m, 6H)
Examples 1 to 7 and Comparative Examples 1 and 21
[0083] A solvent obtained by dissolving LiPF.sub.6 in a mixed
solvent in which ethylene carbonate (EC) and ethyl methyl carbonate
(EMC) were mixed at a volume ratio of 3:7 so as to have a
concentration of 1.0 M was used as a basic electrolyte (1M
LiPF.sub.6, EC/EMC=3:7), ingredients shown in the following Table 1
were additionally injected, thereby preparing electrolytes.
[0084] A battery to which the non-aqueous electrolyte was applied
was manufactured as follows.
[0085] After mixing LiNiCoMnO.sub.2 and LiMn.sub.2O.sub.4 at a
weight ratio of 1:1 as a cathode active material, the active
material, polyvinylidene fluoride (PVdF) as a binder, and carbon as
a conductive material were mixed at a weight ratio of 92:4:4 and
then dispersed in N-methyl-2-pyrrolidone, thereby preparing cathode
slurry. This slurry was coated on aluminum foil having a thickness
of 20 .mu.m, dried, and rolled, thereby preparing a cathode. After
mixing artificial graphite as an anode active material,
styrene-butadiene rubber as a binder, and carboxymethyl cellulose
as a thickener were mixed at a weight ratio of 96:2:2 and then,
dispersed in water, thereby preparing anode active material slurry.
This slurry was coated on copper foil having a thickness of 15
.mu.m, dried, and rolled, thereby preparing an anode.
[0086] A film separator made of a polyethylene (PE) material and
having a thickness of 25 .mu.m was stacked between the prepared
electrodes, and a cell was configured using a pouch having a size
of 8 mm.times.270 mm.times.185 mm
(thickness.times.length.times.width), followed by injection of the
non-aqueous electrolyte, thereby manufacturing a 25 Ah-class
lithium secondary battery for an electric vehicle (EV).
[0087] Performance of the 25 Ah-class battery for an electric
vehicle (EV) manufactured as described above was evaluated as
follows. Evaluation items are as follows.
[0088] *Evaluation Item*
[0089] 1. Capacity Recovery Rate after 30 days at 60.degree. C.
(storage efficiency at a high temperature): A battery was charged
to 4.4V with 12.5 A CC-CV at room temperature for 3 hours, left at
60.degree. C. for 30 days, and discharged to 2.7V with CC with 25 A
current, and thereafter, available capacity (%) relative to initial
capacity was measured.
[0090] 2. Thickness Increase Rate after 30 days at 60.degree. C.: A
battery was charged to 4.4V, with 12.5 A CC-CV at room temperature
for 3 hours, and thereafter, a thickness of the battery was
indicated as A, and a thickness of the battery left at 60.degree.
C. by using a closed thermostatic device for 30 days under normal
pressure exposed to atmosphere was indicated as B, then a thickness
increase rate was calculated by following Equation 1:
(B-A)/A*100(%) [Equation 1]
[0091] 3. Life cycle at Room Temperature: A process of charging the
battery at room temperature (25 A, 4.4V, CC-CV) for 3 hours and
then discharging the battery to 2.7V (25 A) was repeated 300 times.
In this case, discharge capacity at a first time was defined as C,
and discharge capacity at a 300th time was divided by the discharge
capacity C at the first time, thereby calculating a capacity
retention rate during the life cycle.
[0092] 4. 1 C Discharge at -20.degree. C. (discharge efficiency at
a low temperature): After the battery was charged at room
temperature for 3 hours (12.5 A, 4.4V, CC-CV), the battery was kept
at -20.degree. C. for 4 hours, and then the battery was discharged
to 2.7V (25 A, CC). Then, usable capacity (%) with respect to
initial capacity was measured.
TABLE-US-00001 TABLE 1 Capacity After 30 days Retention at
60.degree. C. Rate Capacity Thickness during Discharge Recovery
Increase Life Capacity Electrolyte Composition (100 wt %) Rate Rate
cycle (-20.degree. C.) Example 1 Basic Electrolyte + PHE22 10 wt %
86% 5% 87% 79% Example 2 Basic Electrolyte + PHE24 10 wt % 84% 9%
84% 76% Example 3 Basic Electrolyte + PHE25 10 wt % 75% 10% 76% 71%
Example 4 Basic Electrolyte + PHE31 10 wt % 83% 9% 86% 80% Example
5 Basic Electrolyte + PHE22 10 wt % + LiBOB 89% 2% 89% 84% 1 wt %
Example 6 Basic Electrolyte + PHE22 10 wt % + VC 1 wt % 90% 1% 92%
83% Example 7 Basic Electrolyte + PHE22 10 wt % + VC 1 wt 93% 1%
91% 79% % + PS 1 wt % Comparative Basic Electrolyte 37% 30% 20% 55%
Example 1 Comparative Basic Electrolyte + CN--(CH.sub.2).sub.4--CN
10 wt % 65% 16% 68% 12% Example 2 Basic Electrolyte: 1M LiPF.sub.6,
EC/EMC = 3:7/ PHE22: Compound of Preparation Example 1/ PHE24:
Compound of Preparation Example 2/ PHE25: Compound of Preparation
Example 3/ PHE31: Compound of Preparation Example 4/ LiBOB:
Lithium-bis(Oxalato)Borate/ VC: Vinylene carbonate/ PS: 1,3-propane
sultone
[0093] As described above, it may be appreciated that the
high-voltage lithium secondary battery containing the electrolyte
for a high-voltage lithium secondary battery according to the
present invention has low-temperature discharge efficiency of 71%
or more and high-temperature storage efficiency of 75% or more.
Further, it was confirmed that in Examples 1 to 7, the thickness
increase rate of the battery at the time of keeping the battery at
a high temperature for a long period of time was significantly low
(1 to 10%) and the capacity retention rate during the life cycle
was excellent (76% or more). On the contrary, in Comparative
Examples 1 and 2, low-temperature discharge efficiency was 55% or
less, high-temperature storage efficiency was 65% or less, and the
thickness increase rate of the battery at the time of keeping the
battery at a high temperature for a long period of time was
significantly increased to 16 to 30%. In addition, in Comparative
Example 1, the capacity retention rate during the life cycle was
20%, and in Comparative Example 2, the capacity retention rate
during the life cycle was 68%.
[0094] In order to measure oxidative decomposition potentials in
Examples 1 to 4 and Comparative Example 1, linear sweep voltametry
(LSV) was measured using a Pt electrode as a working electrode and
a Li-metal as a counter electrode and a reference electrode. As a
result, it was confirmed that in the electrolyte of Example 1
(basic electrolyte+PHE22 10 wt %), the electrolyte of Example 2
(basic electrolyte+PHE24 10 wt.), the electrolyte of Example 3
(basic electrolyte+PHE25 10 wt %), and the electrolyte of Example 4
(basic electrolyte+PHE31 10 wt %), 10 wt % of PHE22, 10 wt % of
PHE24, 10 wt % of PHE25, and 10 wt % of PHE31 were added,
respectively, as compared to the basic electrolyte of Comparative
Example 1, such that oxidation potentials of the electrolytes were
increased, and thus, the electrolytes were less decomposed at a
high voltage (FIG. 1).
[0095] Particularly, comparing Examples 1 to 4 with Comparative
Example 2, it may be appreciated that high-temperature storage
efficiency (Example 1: 86%, Example 2: 84%, Example 3: 75%, Example
4: 83%, Comparative Example 2: 65%), the thickness increase rate of
the battery at the time of keeping the battery at a high
temperature for a long period of time (Example 1: 5%, Example 2:
9%, Example 3: 10%, Example 4: 9%, Comparative Example 2: 16%), and
low-temperature discharge efficiency (Example 1: 79%, Example 2:
76%, Example 3: 71%, Example 4: 80%, Comparative Example 2: 12%)
were significantly different depending on structures of nitrile
compounds added to the basic electrolyte. The difference as
described above is due to structural characteristics of the nitrile
compound added to the basic electrolyte, and the multi-nitrile
compound according to the present invention, which has a structure
in which at least three of four substituents substituted at a
central carbon atom are substituents except for hydrogen, and at
the same time, at least two thereof are nitrile groups, that is,
cyanoalkyl groups or cyanoalkyloxyalkyl groups, has a different
structure from adiponitrile
(CN--CH.sub.2CH.sub.2CH.sub.2CH.sub.2--CN) of Example 2 in which
hydrogen atoms are introduced at all carbon atoms present between
two nitrile groups.
[0096] That is, in an adiponitrile compound of Comparative Example
2 having a structure in which the carbon atoms present between two
nitrile groups are not substituted with other substituents except
for hydrogen, atoms may freely move in molecules, such that
permittivity may be relatively decreased, and thus, it is
impossible to effectively block a cathode surface. Therefore, in
Comparative Example 2 in which a compound having a linear aliphatic
hydrocarbon group introduced therein, in which another substituent
except for hydrogen is not introduced between two nitrile groups,
was added to the electrolyte, the electrolyte may be easily
oxidized and decomposed at a high voltage, which may cause side
reactions in the battery. Therefore, high-temperature storage
efficiency and low-temperature discharge efficiency may be
decreased, and the thickness increase rate of the battery at the
time of keeping the battery at a high temperature for a long period
of time may be increased. In addition, it may be confirmed that
low-temperature discharge characteristics of the nitrile compound
of Comparative Example 2 (adiponitrile) were further deteriorated
as compared to Comparative Example 1 in which a nitrile compound
was not added.
[0097] However, the electrolyte for a high-voltage lithium
secondary battery according to the present invention contains the
multi-nitrile compound having a structure in which at least three
of four substituents substituted at a central carbon atom are
substituents except for hydrogen, and at the same time, at least
two thereof are nitrile groups, that is, cyanoalkyl groups or
cyanoalkyloxyalkyl groups, such that side reactions in a battery
may be suppressed. Therefore, a swelling phenomenon of the battery
due to oxidation/decomposition of the electrolyte in a high voltage
state may be significantly decreased, such that the battery may
have excellent discharge characteristics even at a low temperature
as well as excellent high-temperature storage characteristics.
[0098] Although the exemplary embodiments of the present invention
have been disclosed in detail, those skilled in the art will
appreciate that various modifications are possible, without
departing from the scope and spirit of the present invention as
disclosed in the accompanying claims. Accordingly, such
modifications of the embodiment of the present invention should
also be understood to fall within the scope of the present
invention.
INDUSTRIAL APPLICABILITY
[0099] An electrolyte for a high-voltage lithium secondary battery
according to the present invention contains a multi-nitrile
compound having a structure in which at least three of four
substituents substituted at a central carbon atom are substituents
except for hydrogen, and at the same time, at least two thereof are
nitrile groups, that is, cyanoalkyl groups or cyanoalkyloxyalkyl
groups, such that a swelling phenomenon of a battery due to
oxidation/decomposition of the electrolyte in a high-voltage state
may be significantly decreased, and thus the electrolyte may have
excellent discharge characteristics even at a low temperature as
well as have excellent high-temperature storage
characteristics.
[0100] Further, a high-voltage lithium secondary battery containing
the electrolyte for a high-voltage lithium secondary battery
according to the present invention may significantly decrease a
swelling phenomenon of the battery due to oxidation/decomposition
of the electrolyte at a high voltage state while properly
maintaining basic performance such as high efficiency charge and
discharge characteristics, life cycle characteristics, and the
like, thereby having discharge characteristics at a low temperature
as well as excellent high-temperature storage characteristics.
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